We are refinishing a 100 yr old house. The woodwork is wide and detailed, believed to be shellacked. Some of it is in good shape, but some is weathered and cracked. Is it a good idea to use a sanding sponge and scuff down some of the finish and re-coat with a polyurethane or do we have to strip it down completely? What type of polyurethane would you recommend? The wood is dark, so can polyurethane be tinted?